Vue3 + Typescript - Vue warn: Component is missing template or render function

Question

I saw this topic has a lot of questions, but I really didn't find any useful answer for my case.

I'm importing 3 components in my App.vue file. 2 out of 3 are working fine, but "FrequencyBox.vue" get the following warning and doesn't render at all:

[Vue warn]: Component is missing template or render function. 
  at <FrequencyBox> 
  at <App>"

App Component:

<template>
  <h1>Sono App.vue</h1>
  <Header />
  <FrequencyBox />
  <Footer />
</template>

<script lang="ts">
import { defineComponent } from "vue";
import Header from "./components/Header.vue";
import FrequencyBox from "./components/FrequencyBox.vue";
import Footer from "./components/Footer.vue";

export default defineComponent({
  name: "App",
  components: {
    Header,
    FrequencyBox,
    Footer,
  },
});
</script>

<style>
#app {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>

FrequencyBox component:

<template>
  <h1>Sono la Frequency Box</h1>
</template>

<script lang="ts">
import { defineComponent } from "vue";
export default defineComponent({
  name: "FrequencyBox",
});
</script>

<style></style>

Any ideas?